Astronomer Misty Bentz would like you to know that black holes don't suck. "They're not cosmic vacuum cleaners going around and sucking everything in," she says. "They just use gravity the same way everything else does."
Instead of a cosmic drinking straw, a black hole is a place in the universe that is so massive and dense that anything caught in its significant gravitational pull is unable to escape.
Black holes have been in the news a lot lately, from the swarm found near the supermassive black hole at the center of our galaxy, to the fastest-growing black hole ever observed, which ingests the equivalent of the mass of our Sun every two days, to the most distant black hole ever detected, dating to the dawn of the universe. From their inception to their potential demise far in the future, black holes are a fascinating part of our universe. Here's their story as we understand it now, from start to finish.
A stellar-mass black hole starts its life with a death. It's born when a star at least 10 times more massive than our Sun runs out of fuel, having already fused hydrogen into helium, and helium into other elements, from carbon and oxygen all the way up to iron deep in the star's core. With a weighty metal heart it has nothing left to bind together. It's reached the end of its lifespan, and it explodes, sending the outer layers out in a violent burst as the core collapses in on itself.
“If there’s enough mass there—three times the mass of the Sun in the center of the star—this will collapse into a black hole. We call those stellar mass black holes because they have a mass similar to a star,” says Bentz, an astronomer at Georgia State University.
This link between the birth of a black hole and the death of the star that formed it is a fairly common occurrence across the Universe. Stars and black holes are closely intertwined, especially in areas of the universe where star formation is churning along at high speed.
“It's actually really common to find dead stars where the new stars are forming, because the most massive ones don’t live very long. They’re gone right away,” Bentz says. “The lifetime of a star depends on its mass. The most massive stars live much shorter lives because they just burn through their fuel very quickly.”
In what Bentz calls a ‘giant recycling program,’ the creation of black holes can actually spark the formation of new stars as well. When a group of new stars form, the most massive among them die out very quickly, exploding at the end of their short lives. “Those shockwaves compress more gas and dust to cause more stars to start forming. Then the most massive of those will live short lives and explode, which will send out new shockwaves and start forming more stars. It's this chain reaction of the deaths of stars causing the births of brand new stars,” Bentz says.
But stellar-mass black holes are only a small part of the picture. Much weirder are supermassive black holes, giant beasts whose origins are far more obscure. They've been observed at the center of galaxies, including our own, and seem to have a slightly different way of forming than their smaller compatriots.
“A supermassive black hole as we see it now has a mass of a million or a billion times the mass of the Sun. But it didn’t start that way, it started smaller. So the question is, how did they form and how did they get that big?” Jillian Bellovary, a theoretical astrophysicist at Queensborough Community College says.
Astronomers know that supermassive black holes got really big, very fast, showing up around 13 billion years ago. At that point, Bellovary says, “we already see that there are black holes that have billion times the mass of the Sun. We know they existed really early in the universe, and that's weird because there is all this mass in a very small space, and we want to know how it got there.”
“It's a bit of a chicken or the egg kind of problem,” Bentz says. “In the very early universe it's possible we formed black holes just from direct collapse of really over-dense regions. Maybe the material started collapsing gravitationally, and then kept collapsing all the way down into a black hole and didn’t actually form stars or anything.”
The other option is that maybe supermassive black holes got their start in early galaxies, as smaller black holes formed and coalesced in the center of infant galaxies.
The precursors to these early supermassive black holes were likely moderately sized to start with, says Bellovary, and would have had to be larger than a mere stellar mass black hole, which wouldn’t have been able to grow fast enough in such a short period of time to form the behemoths of the early universe that we’ve observed.
“The supermassive black hole has to get some sort of jump start, it can’t be too small when it forms because then it won’t have enough time to get huge. So it has to be medium-sized when it forms.” Bellovary says.
Researchers are still trying to figure out how those first black holes would have formed from the hot gas and dust of the early universe. Typically, when matter like that collapses together, it forms stars. So there may have been something different about the chemistry of the early universe that helped kindle those initial black holes.
"That gas in the early universe was probably only made of hydrogen and helium, because those were the only elements made in the Big Bang, and everything else was made inside of stars. If you don't have stars yet, you can't have any other elements yet," Bellovary says. The chemistry of the early universe, as well as the motion, or lack of motion of the gas could have helped trigger black hole formation in those early eons.
Black holes don’t just stay at the same size forever. They get their (undeserved) sucky reputation because things that fall into them can never get out, and are instead added to the collective mass of the black hole, letting it grow.
“It doesn’t matter if it's gas that’s falling onto it, If it's another star that gets ripped apart and falls onto it, if its a planet that got ripped apart and fell onto it—whatever goes in adds to the mass of the back hole. That accretion process, eating little bits of stuff over a long period of time, that’s one way that black holes grow over the history of the Universe.” Bentz says.
“The way we think they can grow most efficiently is by swallowing up gas or accreting gas,” Bellovary says. “Gas falls into a black hole like water drains down in a bathtub: it swirls around and goes into a drain. Gas acts similarly in a black hole. It’s gravitationally attracted to the black hole, but it's moving, so it starts to thin into a disc around the black hole and eventually, it falls in.”
Acquiring gas might be the most efficient way to grow, but black holes don't shy away from mergers. Collisions between black holes end with the two invisible masses uniting, something that scientists can observe with the Advanced Laser Interferometer Gravitational-Wave Observatory, or LIGO, which first detected gravitational waves from the merger of two black holes in 2015 (the announcement of the discovery came in 2016).
“For the first time we can learn things about the universe that don’t involve light. We’ve always been dependent on light and our eyes before this. Without light we wouldn’t know anything about the Universe. Light has been wonderful to us, but for the first time we can now see things that would be impossible to see with light, like merging black holes,” Bellovary says. “We would never know what happens without gravitational waves.”
There's a huge size gap between supermassive and stellar mass black holes, where by all accounts there should be intermediate black holes—medium-sized, Goldilocks-approved black holes that fit just right in between their smaller and larger cousins.
The only problem is that researchers haven’t observed them yet.
That doesn't mean that intermediate black holes don't exist, and many researchers are actively looking for them. But the're hard to detect with visible light, unlike nearby stellar-mass black holes, which astronomers can observe ripping apart stars, or supermassive black holes, which acquire so much gas and dust and mass that the colliding particles falling in shine brighter than anything else we've seen in the Universe.
“When it comes to intermediate black holes that don’t have as much gravitational force, they don’t tend to shine as brightly,” Bentz says. They also are expected to shine more in x-ray wavelengths of light, and there are already other objects in the Universe that dominate that particular spectrum.
“It's a little hard to tell them apart from other things,” Betnz says. “They’re probably out there, it's just that we’re having a hard time finding the ones where we can say ‘Oh, sure, this is definitely the one this time and everyone can believe it now.’”
Whatever the size, black holes go through certain phases—coming into being, growing—throughout the course of their existence. But can they ever die? Stephen Hawking thought that it might be possible, through a physics mechanism that is now known as Hawking radiation.
The idea is that if a black hole were sitting there by itself (no longer accreting mass) it could eventually be worn down by subatomic particles. Bentz explains that it goes something like this: all over the universe, pairs of subatomic particles are popping into existence right next to each other. One half of the pair is a particle, the other is an antiparticle, and usually just after they spring into the Universe, they smack into each other and vanish into energy again.
“It's just energy converting to mass converting back to energy, popping in and out of existence,“ Bentz says. “If that happens near a black hole and one-half of that pair is inside the black hole and one is outside the event horizon and the one outside can get away from the event horizon—well, then it has stolen a little bit energy from the black hole, and it can run off and take that energy away.”
If that process keeps happening repeatedly without more mass joining the black hole, eventually you can radiate the whole thing away. But for black holes—whether stellar mass or supermassive or anything in between—it would take huge amounts of time to even make a dent.
“There hasn’t been enough time in the Universe yet for a black hole to die, even if you were to create one at the very beginning of the Universe. It's going to take something like 10^54 years before the first black holes start dying,” Bentz says.
The incredible amount of time and the huge difference in scale between a massive or supermassive black holes and subatomic particles means that the slow leak of Hawking radiation from a black hole is impossible to observe directly. Experiments in laboratories on black hole analogs indicate that Stephen Hawking's theory was probably right, but there's still a lot we don't know about the potential end of a black hole's existence.
In fact, there’s still plenty we don’t know about black holes in general, but researchers like Bentz and Bellovary are working to fill in those gaping voids in our understanding.
Bentz is looking into just how massive the supermassive black holes at the center of distant galaxies can get. She’s also researching correlations between the size of black holes and the properties of the galaxies that they live in. She hopes that her observations could help inform computer models that help people figure out how black holes develop over time, and answer questions about the formation of the universe.
Bellovary is working on the Laser Interferometer Space Antenna, or LISA, a mission planned for the 2030s which will feature three spacecraft situated a gargantuan 2.5 million kilometers apart following behind Earth's orbit. It will be a space-based gravitational wave detector—like LIGO, but focused on supermassive black hole mergers instead of stellar mass black hole mergers.
“I’m really excited about this because it will give us insight into things that we can’t ever see with light and teach us how black holes grow, how often do they merge with each other, do they get mass from colliding with each other or accreting gas, how do they form, how many of them are there, are there some in places where we can’t see them—all sorts of things that we would never be able to answer using light,” Bellovary says.
Black holes are intriguing, huge, and still very unknown, but there's one thing that researchers are certain of. “They are not dangerous. There aren’t any that are close to us, and we don’t need to worry about them,” Bellovary says. “There are many, many other things to be worried about.”
Bentz emphatically agrees. “They are extreme, but they are not scary,” she says. When facing down people’s fears that black holes are running around the galaxy devouring worlds and star systems, she likes to point out that if you replaced our Sun with a black hole of the exact same mass, not much would happen to Earth’s orbit. “The Earth would just keep orbiting around it as it always has before. It would just get really dark and cold. We would have to fall into the black hole in order to be gobbled up—and if we fell into the Sun, that would be really bad, too.”
“Black holes are so far away from us that they’re not dangerous. It’s only [dangerous] if you get up close to one—then it's too late for you.” Bellovary says.
